Method of comminuting materials by autogenous grinding in a continuous grinding mill



y 1965 A. B. NOREN ETAL 3,181,800

METHOD OF COMMINUTING MATERIALS BY AUTOGENOUS GRINDING IN A CONTINUOUS GRINDING MILL 2 Sheets-Sheet 1 Filed Sept. 25, 1961 M y 1965 A. B. NOREN ETAL METHOD OF COMMINUTING MATERIALS BY AUTOGENOUS GRINDING IN A CONTINUOUS GRINDING MILL 2 Sheets-Sheet 2 Filed Sept. 25,- 1961 INVENTORS n mm mm mh m 7 2% e H, b 5 m s m 3 3 Amumw r AIM United States Patent 3,181,800 METHOD OF COMMINUTING MATERIALS BY AUTOGENOUS GRINDING IN A CONTINUOUS GRINDING MILL Anders Bertil Norn, Djursholm, and Per Anders Herman Henningsson Fahlstriim and Jonas Columbus Burstriim, Boliden, Sweden, assignors to Bolidens Gruvaktiebolag, Skelleftehamn, Sweden, a joint-stock company limited of Sweden Filed Sept. 25, 1961, Ser. No. 141,261 Claims priority, application Sweden, July 26, 1957, 6 ,97 7 7 2 Claims. (Cl. 241-24) This is a continuation in part of our application Serial No. 748,514, filed July 14, 1958, and now abandoned.

The object of this invention is to provide an efiicient method for bringing the particle size of the finished ground product to a predetermined value continuously in a particle size reduction system in which solids, in particular run of mine ore, of varying dimensions While comminuting themselves are used as crushing and grinding bodies, whereby the eifect of variations in the dimensions of the solids fed to the mill on the particle size of the finished ground product is compensated for.

A further object of the invention is to provide a method for adjusting the mesh of grind of the finished ground product by automatically adjusting the mill speed.

Size reduction of solids by grinding with the use of the solids as crushing and grinding bodies is conventionally defined as autogenous or rock grinding. Despite the fact that in the process of the present invention the material is subjected to comminution over a size range that is normally obtained by both crushing and grinding steps, the comminution process of the present invention for convenience will be in the following description and claims referred to as autogenous grinding.

Up to the present time the grinding of solid materials to a great extent has been carried out in rod mills, ball mills and tube mills. These mills are charged with a loading of grinding bodies adapted to the size of the material fed to the mill, which size usually is approximately 1 /2" to l or less, as well as the desired fineness of the finer grinding bodies when a product of higher fineness is de-' sired. The grinding operation is then carried out in several stages in each of which the size of the grinding bodies is adapted to the successive reduction of the particle size.

This, in conventional ball mills it is possible to a certain degree to control the fineness of the finished product by choice of the size of the grinding balls. However, such a control cannot be effected to overcome sudden particle size variations of short duration of the feed, as it requires several weeks of operation to markedly change the grinding characteristics of a steel ball charge during the change of size of the new balls added to the mill.

Furthermore, in grinding systems utilizing grinding media of external material the materials to be ground must be subjected to a primary crushing which requires a separate apparatus. In addition to that, in these mills the grinding media usually consist of steel balls which are worn in proportion to the material passed through the mill. Charges of hard materials to be ground are more wearing than easily ground materials, and grinding to a higher fineness consumes more steel than grinding to a lower fineness.

It is well known to use screened pieces of defined size of the non-ground material to replace the grinding bodies used in tumbling mills, for instance balls in ball mills or flint balls in tube mills. Of greater interest is that kind of autogenous grinding, in which the solids, after no or a less particle size reduction in crushers, are fed to a rotating grinding shell, in which they are comminuted to a 3,181,800 Patented May 4, 1965 ICC desired particle size, due to the reciprocal crushing and abrading action of the solids on each other. This latter method has been referred to as run-of-mine ore grinding. Autogenous grinding according to the run-of-mine ore grinding method is usually carried out in mills of short axial length and large diameter and is elfected as a wet as well as a dry grinding process. The method is of great importance, primarily for the grinding of ores for subsequent concentration, rocks, products within the cement industry, coke and so on. In these cases a substantial part of the equipment necessary for crushing and grinding by conventional means are saved. Further the costs for grinding bodies of external material are saved, these costs usually being considerable.

The adjustment of the particle size of the finished product has been brought about up to now only by suitably adapting the size grading and distribution of the material fed to the mill or by varying the weight of feed per unit of time.

This is carried out in several ways, for instance by means of a certain crushing step, whereby a certain maximum particle size and a defined size distribution are im parted to the material. It is also known to screen out certain size fractions from the feed and to recombine these fractions with the other fractions in a constant ratio in order to achieve a defined size distribution of the feed. Further it is known to screen out a fraction from the material, that is coarser grinding bodies, and subject the balance of the material to crushing and in a subsequent grinding operation recombine the two parts in a constant ratio. The reason for these different known methods for adjusting the size grading and distribution ofthe feed is that they have a predominant effect on the screen analysis or mesh of grind (MOG) of the finished product.

Still another method of controlling the mesh of grind of the finished product is to vary the tonnage fed to the mill per unit of time. When, however, which is usual, the mill is the primary step in a series of production steps, a variation in the tonnage fed to the mill per unit of time bears effect, usually undesirable on the entire production. To overcome this disadvantage, a reservoir has to be arranged between the grinding unit and next production unit. In the case the ground material can be stored this means an additional cost while in other cases the ground product cannot be stored at all due to ageing before processlng.

In order to obtain a certain desired particle size of the ground product when continuously feeding the run-ofmine material to a rotating mill of fixed rpm. and comminuting the material, the feed is required to have a certain size grading and distribution. If so, a suitably graded charge of tumbling bodies are formed in the mill, which is a necessity for obtaining a defined particle size of the finished product. It the material fed to the mill drum will cause an increase in number and a decrease in average size of the bodies forming the charge tumbling in the mill drum, a larger mass will be tumbling in the mill at a lower speed. Correspondingly, an increase of the rpm. of the mill drum will cause a decrease in number and an increase in average size of the mass of bodies tumbling in the mill drum. Due to this a very close control of the feed grading and distribution is necessary in all methods mentioned in the foregoing, which limits the applicability of the methods.

The main object of the present invention is to provide an efiicient autogenous grinding system for continuously bringing the mesh of grind (MOG) of the finished product to a predetermined value, in which the material consisting of an arbitrary mixture of coarse and fine solids, for instance a feed comprising run-of-mine ore, is used as crushing and grinding bodies without grinding media of external material present, whereby the effect of variations in the dimensions of the solids fed to the mill on the mesh of grind of the finished product is compensated for.

The inventors have found that if the material fed to the mill attains a size composition which gives a coarser finished product the desired screen analysis can be restored to its desired value by decreasing the rotational speed of the mill drum and that, if the material fed to the mill is undergoing such variations that a product finer in size than that desired is discharged from the mill, the screen analysis of the product can be restored to its original value by increasing the rotational speed of the mill drum. Of course, this method of controlling the screen analysis or MOG of the finished product is feasible only if the mill drum is not utilized to its maximum loading capacity. As mentioned hereinabove, at a constant weight of feed per unit of time a decrease of the rotational speed of the mill drum will cause an increase in number and a decrease in size of the grinding bodies tumbling in the mill drum and a larger mass to be tumbling in the mill at a lower speed and that, correspondingly, an increase of the rotational speed of the mill drum will cause a decrease in number and an increase in size of the grinding bodies tumbling in the mill drum. In this respect the autogenous grinding mill differs from conventional mills operating with ordinary grinding bodies such as steel balls and rods and the like. In a conventional mill undergoing a decrease in its rotational speed the mill charge does not change its average size by itself but the grinding capacity of the mill decreases roughly in proportion to the reduction in the number of revolutions. Contrarily, the grinding capacity of the mill increases roughly in proportion to an increase in the number of revolutions. ,The fact that the size reduction sometimes is believed to be more effective at lower numbers of revolutions than at higher ones, for instance expressed as new surface tons per kW.-hr. does not appear directly to reflect itself on the present invention the cardinal adjusting step of which implying an adaption of the composition of the charge tumbling in the mill drum to give a finished product of the desired MOG or screen analysis.

The method according to the invention comprises the steps of feeding the material to be ground to the mill at a substantially predetermined weight per unit of time, comminuting said material by the autogenous grinding action caused by the interaction of the tumbling mass of bodies to be ground, continuously discharging the ground product from the mill, continuously controlling the screen analysis of the finely ground product by increasing the r.p.m. of the mill drum when the particle size of the finely ground product is finer than a predetermined screen analysis, and decreasing the rpm. of said mill drum when the particle size of the finely ground product is coarser than said screen analysis.

The rotational speed of the mill for the adjustment is suit-ably selected at a value below the critical speed of the mill, for instance between 50 and 100%. The critical speed of a mill is defined as that peripheral speed of the cylindrical inner wall of the mill shell at which a particle located at the cylindrical inner wall is caused to be carried with it to the highest point thereof. The critical speed (n is expressed in rpm. Thus, the operational speed of the mill according to the present invention is .5 to 1.0 times the critical speed, the absolute value of n being dependent on the inside diameter of the mill shell according to the equation n 76.5 eritwhere- D is the inner diameter of the mill in feet.

More specifically, the method for continuously cntrolling the screen analysis according to the present invention comprises continuously sampling the ground product, feeding the samples obtained to a testing screeen having a mesh width corresponding to a particle size within the range of the screen analysis of the finished product, screening said samples to produce an oversize fraction and arr undersize fraction, continuously producing electric signals proportional to the Weights of said fractions, continuously comparing said two signals to produce a first difference signal, continuously producing a reference signal corresponding to the nominal weight ratio of the predetermined oversize and undersize fractions according to the desired screen analysis of the finished product, comparing said first difference signal with said reference signal to produce a second difference signal, and varying the rotational speed of the mill in accordance with the sense and magnitude of said second difference signal.

It will be appreciated that when said second difference signal attains the value zero, the finished product has the MOG desired. If said second difference signal attains a negative value the finished product is of a finer MOG, in which case the rotational speed of the mill drum is increased whereas, if said second difierence signal attains a positive value, the finished product is of a coarser MOG, in which case the rotational speed of the mill drum is decreased. The method for sampling the product and producing the electrical signals as well as the method for adjusting the r.p.m. of the mill will be described more in detail in the following.

The invention will now be described and explained in the following with reference to the enclosed drawings. Further features of the invention will be evident from the following description.

FIGURE 1 shows graphically the relationship between the rpm. of the mill and the particle size of the finished product when grinding in a system according to FIG- URE 3.

FIGURE 2 shows schematically a plant with devices for carrying out the process according to the invention.

FIGURE 3 shows schematically a modified plant with devices for wet grinding of ore.

FIGURE 4 shows schematically a device for continuously testing the screen analysis of the product discharged f-rom the mill and automatically controlling the rpm. of the mill to compensate for deviations from a predetermined screen analysis.

Referring now more particularly to the drawings, FIG- URE 1 is a graph showing the influence on the particle size of the product in grinding a lead ore. As an in dependent variable the mill speed, expressed in terms of percent of the critical speed, has been plotted along the X-axis and as a dependent variable the screen analysis of the product, defined as mesh of grind or MOG. Each curve represents a certain feed composition and size distribution, curves a to c representing feed ore of gradually coarser size. By simplicity this is expressed in terms of percentage of material being coarser than 3 /2 inches, curves a, b and c representing values of 20, 40 and 60%. For the man of the art it is well known that although a material comprises a number of particles of different sizes it is convenient to select one as a standard reference to which all the other can be linked. The most commonly employed method is to define the so-called mesh of grind or MOG, which according to Taggart, Handbook of Mineral Dressing, Section 4 page 10 is defined as the theoretical square mesh apertures that would pass percent of the product. If the sieve aperture is defined by the edge length of the squares forming a sieve, the mesh of grind has a physical sense. In the description for convenience the particle size of the ground product or the MOG is used with the definition given hereinabove.

The curves represented in the graph of FIGURE 1 refers to a lead ore which consists essentially of a quartzitic sandstone with intergrowths of galena assaying in average 6 percent Pb. The ore as mined after having been subjected to a very light primary crushing was fed to the mill with a maximum lump size of 15 inches and a normal distribution of all sizes down to micron size, however, due to varying ore conditions and mining method showing a substantial variation in size distribution of pieces of different sizes, represented by curves a to 0, FIG- URE 1.

The results were obtained when grinding in a mill having an inner diameter of and an inner shell length of 3 and with a charge volume of approximately 35%. Substantially the same curve has been obtained in a 22 x 7' mill.

According to the inventors the controlling etfect on the grinding achieved by the invention is probably due to the observation that grinding charges of different size distribution are formed at different speeds. Thus, at a low speed there is formed a charge consisting of a larger number of grinding bodies having a relatively large surface area. At higher speeds a more rapid crushing of the grinding bodies occurs, which primarily hits the smallest pieces, so that the number of effective grinding bodies and as a result thereof also the surface of the grinding bodies decrease even if the total weight of the charge is equal the same. The invention is particularly useful in run-of-mine ore grinding the maximum lump size of up to approximately inches being limited only with respect to the size of the trunnion feed opening of the mill and the means for handling and transportation of the ore to the mill.

In FIGURE 2 a mill is shown, consisting of a drum 1 having a substantially horizontal axis of rotation. The drum comprises a cylindrical shell 2 and frusto-conical or outwardly bulged end walls 3, 4, so that the length of the drum increases towards its centre. The grinding drum is provided with two hollow trunnions 5, 6 by means of which it is mounted for rotation in two bearings 7, 8, each placed on its foundation 9. The driving mechanism of the mill comprises a gear rim 10 attached to trunnion 6 outside the bearing 8, and meshing two pinions 11 mechanically connected with gear boxes 12 the input shafts of which are in turn each connected with a driving motor 13. The motors are preferably electric D.C. motors the rotational speed of which can be regulated continuously, for instance according to the Ward-Leonard system.

The Ward-Leonard system, which is a known method of speed control for large D.C. motors, comprises a D.C. motor (i.e. the motor the rotational speed of which is to be regulated) the field winding of which is connected in series to a D.-C. generator. The output of the generator is regulated by means of the field winding energizing current which is supplied from a separate auxiliary generator, the armature of which is usually mechanically coupled to the main generator. The two D.C. generators are driven by a common A.-C. motor. The rotational speed control of the main D.C. motor is accomplished byvarying the energizing current supplied to the field winding of the main generator, said energizing current being controlled by means of a rheostat controller connected in series in the circuit. The system provides for a speed control of a very high efiiciency within a wide range of r.p.m. and loads. Outside the gear rim 10 the hollow trunnion 6 serving as an overflow for the ground product has an extension ending in a chute 14.

The inner wall of the grinding drum is provided with a wear-resistant steel lining 15, 16, 17. The end wall 4 at the outlet side of the drum is only partly lined, more specifically at the peripheral part so that the lining 15 terminates in an edge surface. From said edge surface an annular plate 18 with grates 19 mounted in a spaced relation to and parallel with the end wall 4 extends towards the centre and as a continuation of the lining 17. In this way there is formed a space between the grate and the end wall, which space by means of a number of radially extending partitions 20, serving as scooping members, is divided into a corresponding number of sec- .7

tor-shaped chambers. Said partitions 20, also act as wear plates to protect the inner-wall of the end wall 4. Towards the centre the grate 19 terminates in an outlet funnel 21 of solid material. The spacing of the rods of the grate is generally 8-20 mms. The ratio of shell length to diameter of the grinding drum shown is about 1:3, but according to the invention this ratio may vary within a wide range and is not critical for the operation of the mill. Moreover, the power transmission between the mill drum and the motor or motors may be effected in a different way. According to the invention it is not either necessary that the mill drum is journalled on horizontal trunnions but also mills having vertical trunnions and bein arranged for rotational or gyratory movement may be employed. Into the hollow trunnion 5 serving as a feed opening issues a feed hopper 22. At the upper part of the feed hopper 22 there is arranged the discharge roll 23 of a belt conveyor 24 for feeding ore to be comminuted from a storage bin 25. Between the bin 25 and the belt conveyor 24 there is mounted a weighing device 26. Below the funnel-shaped part 4 of the hollow discharge trunnion 6 is arranged a chute 27, from which the finished product is fed to further stages of the grinding circuit.

The mill is provided with suitable means for separating finished product and returning coarse not finished material to the mill. In the example there are shown devices, which comprise a pump or a fan 28 and a hydraulic cyclone or a cyclone air classifier 29 to be used in wet grinding are to be used in dry grinding, respectively. In wet grinding also other classifiers can be used, for instance spiralor rake-classifiers, hydro-separators etc. The cyclone shown is provided with an inlet opening 31'? for the product to be classified, and an outlet opening 31 for discharging finished product and an opening 32 for discharging such material which has not been ground sufiiciently. The latter material is returned to the feed chute 22 by known means.

The mill is operated as follows:

The charging material to be ground which may either be an ore screened to different fractions and subsequently remixed in a certain ratio, or unscreened ore as mined, is fed by the belt conveyor 24 from the bin 25 to the mill drum 1 at an approximately constant feed rate, which is adjusted to a desired value and recorded in the weighing device 26. In this operation the ore has been subjected only to a minor crushing action to break the chunks. The mill 1 is rotated through the motors 13 via the gear boxes 12 and pinions 11 meshing the gear rim 10. The r.p.m. of the mill has now been adjusted in such a manner that it is adapted to the size of the ore feed as well as to the degree of grinding desired, which in the described case corresponds to an r.p.m. of 70% of the critical speed, when comminuting a feed ore of overage composition and size distribution corresponding to curve b, FIGURE 1. The ore falls through the feed hopper 22 via the hollow trunnion 5 into the grinding chamber and is set in motion. As a result of the rotation of the mill drum the ore is tumbled and is caused to act as crushing and grinding bodies crushing, grinding and abrading themselves. Furthermore, by the rotational movement of the mill a continuous discharge of material takes place through the grate 19 by means of the scooping members 2% which, because of the rotation of the mill, lift the material and then discharge it through the open space between the outlet funnel 21 and the scooping members and therefrom to the discharge trunnion 6. Depending on the interspace of the grate which generally varies between 8 and 20 mms. the discharged, partly ground product contains particles of up to -30 mms. edge size. The material discharged from mill trununion falls into the chute 27 and is thereafter fed to further stages of the grinding circuit.

When the mill operates under conditions of balance grinding to a finished product of the desired mesh, part 7 of the product discharged from the mill and conveyed by fan (or pump) 28 to cyclone separator 29 will settle at its bottom outlet 32 and passed back to the feed hopper 22 at the feed end of the mill drum, whereas the finished product leaves the cyclone through the top oulet 31.

The system shown in FIGURE 2 is adapted for dry as well as for wet grinding. FIGURE 3 is a diagrammatic representation of a modification of the grinding circuit of FIGURE 2 and adapted for autogenous wet grinding of, for instance, ores. In FIGURE 3 reference numerals 1-27 correspond to those of FIGURE 2. In the grinding circuit of the system according to FIGURE 3 there is included a classifier of the so called spiral type the structure of which is not critical for the purpose of the present invention. The spiral classifier is provided with an outlet 33 for the finished product and another outlet 34 for semiground material. In the classifier there is accommodated a rotatable spiral 35 driven by an electric motor 36 over a gear transmission 37.

The mills shown in FIGURES 2 and 3 described so far are known per se.

In FIGURE 4 there is shown an apparatus according to the invention for continuously screening samples of the product discharged from the mill trunnions 6 in an oversize and an undersize fraction, determining the weights of said fractions, determining the weight ratio thereof, comparing the value so obtained with a reference value corresponding to the nominal weight ratio of the oversize and undersize fractions of a discharged product having the desired screen analysis or MOG, to produce an impulse which is imparted to a controller for adjusting the r.p.m. of the drive motor or motors of the mill in a sense to compensate for deviations from the nominal or desired screen analysis or MOG of the product and thus to restore the balance of the grinding circuit.

The apparatus substantially consists of a vibrating testing screen of a known type comprising a screen box 38 having a 2l0-micron (No. 70) screen 39 approximately x 30" in dimensions. A SO-cycle A.-C. vibrator 40 of a known type is secured at a cross beam extending between the two longitudinal walls of the screen box 38. The screen box is suspended by suspension rods 41 attached to helical springs 42. Slope of the screen surface is about to The angle between the vibrator and the screen box is about 60. A chute 43 adapted to pass samples of the finished product discharged from the mill to the vibrating testing screen terminates at the uppermost end of the screen surface. A discharge hopper 44 is mounted beneath the screen box to collect the oversize and undersize fractions separated by the screen. Hopper 44 at its bottom terminates in a chute 45 adapted to collect the undersize fraction of the sampled product. A second chute 46 mounted to the lower wall of the hopper 44 is adapted to collect the oversize fraction of the sampled grinding product flowing over the lowermost edge of the screen through an opening 47 in the hopper wall. A bafiie 48 is arranged to prevent the oversize fraction from entering the undersize fraction space of the hopper. The

two chutes 45, 46 terminate at substantially the same level, for instance /2 to 1", over a horizontally travelling endless belt 49 passing around two pulleys 50, 51 and adapted to distribute the particles of the two fractions in two layers 52, 53 running parallel with each other, it being observed that the discharge ends of the chutes 45, 46 are so arranged with respect to the belt that the two particle layers do not intermingle. The widths of the chutes 45, 46 in a direction perpendicular to the longitudinal axis of the belt conveyor 49 are equally the same so that when the belt is travelling at a constant velocity, by measuring the thickness of the two layers of the particle fractions deposited on the belt, it is possible to estimate the proportion of the two fractions. To this end, there is arranged beneath the upper part of the belt two emitters 54, 55 of electromagnetic or radioactive radiation, for instance based on the principle of gamma-radiation, one 54 arranged beneath the layer of the undersize fraction deposited on the belt and the other 55 arranged beneath the double layer of the undersize and the oversize fractions. It will be appreciated that the extinction of the radiation passing through the two particle layers will be in prop-ortion to the thickness of the layers. Thus, by measuring the residual radiation transmitted through the layers and comparing the resulting values with each other it is possible in a convenient way to estimate the relative Weight proportion of the respective oversize and undersize fractions of the screened samples of the product discharged from the mill, and, furthermore, to compare the value of the actual weight proportion obtained with a reference value relating to the nominal or desired size composition of the finished product to produce a difference value the sense and magnitude of which is caused to adjust the rpm. of the drive motor or motors 13 of the mill drum 1 in a sense compensate for the deviations from the nominal size composition of the discharged product.

To this end, two detectors, 56, 57 adapted to co-operate with the emitters 54 and 55, respectively, are mounted over the conveyor belt in spaced relation to the emitters. By means of conductors 58, 59 the detectors 56, 57 are connected to amplifiers 60 and 61, respectively, the outputs 62, 63 of which are supplied to a calculating device 64. Said calculating device, which is of a known type, is capable of producing a signal proportional to the quotient of the outputs 62, 63. By means of a conductor 65 this signal is supplied to a regulator 66. In said regulator 66 the signal supplied through conductor 65 is compared with a pre-set reference signal supplied through conductor 67. Said reference signal has previously been determined for a mill product of the desired screen analysis specified by a certain ratio of oversize to undersize material of a screened sample dilference or error signal is thus produced in calculator 66 which, by means of conductor 68, is supplied to the speed controller 69 of the drive motor or motors 13 of the mill drum 1.

It should be observed that the 2l0-micron screen mentioned above is only chosen as an example and that the screen obviously has to be chosen with respect to the desired MOG of the product. Since the MOG is defined as the screen aperture through which 80% of the finely ground product passes this implies that 20% are retained on the screen surface and, accordingly, when grinding an ore to the desired MOG the correct or nominal ratio of the weights of the oversize to undersize fractions is 1:4.

The continuous sampling of the product discharged from the mill is effected by means of devices known in the art.

The screening and automatic mill speed controlling unit according to the invention operates as follows.

To obtain the desired MOG of the discharged product (210 micron) the calculator 66 is biased with the specific current 67 previously determined for this MOG and when the system is in balance there will not be produced a difference signal in said calculator. The sampler is set into operation and the vibrator 40 energized. The sample is discharged onto the screen surface through chute 43 and is separated in an undersize fraction and an oversize fraction which are unloaded onto the travelling belt 49 through the chutes and 46, respectively, forming two separate layers 52, 53. The electromagnetic radiation passing from the emitters 54, through the respective layers is extinguished in proportion to the thickness of the two layers. In the detectors 56, 57 there are created currents of a magnitude corresponding to the thickness of the layers of the pulverulent material, said currents being supplied to their respective amplifiers and 61 from which the amplified currents are supplied to the first calculating device 64 in which a first difference current is produced and through conductor 65 supplied to the second calculating device 66 biased by the reference current 67 corresponding to the desired MOG of the finished product. In said second calculator 67 there is produced a difference 9 current which is supplied through conductor 63 to the speed controller 69 of the mill drum 1.

If there is not flowing a biasing current through conductor 68 to the s eed controller 69 this means that the biasing current produced in the first calculator 64 is of the same magnitude and sense as the reference current 67 biasing the second calculator 66 and that there is consequently not produced the second diflerence current which is necessary to actuate the speed control of the mill drum. If, however, the proportion of the oversize and undersize fractions is undergoing variations, there are correspondingly produced dilference currents 6S differing from the current 67 biasing the second calculating device 66, in which case there is produced an output current the sense and magnitude of which is proportional to the deviation from the MOG desired. As a result thereof the speed controller 69 of the mill drum is actuated in a sense to counteract said deviation from the size composition of the product.

It will be appreciated that there are used in the calculators 64 and 66 as well as in the speed controller 69 of the mill drum, thermionic relays and circuitries known in the art and a more detailed description thereof is therefore believed to be superfluous.

The belt conveyor described hereinabove is adapted for deposition of dry solid particles. In the case of wet grinding, however, the belt conveyor consists of an endless filter cloth having three longitudinally extending rubber edgings fastened thereto in spaced relation to collect two layers of equal width of the oversize and undersize fractions of the pulp. Beneath the upper part of the endless filter cloth there is mounted a suction box at a point between the pulp outlets 45, 46 and the thickness measuring devices 54, 56; 55, 57 to drain off the water giving two layers of wet particles. The pulp still clinging to the filter cloth after having passed around the end roller is washed off with Water.

It will further be appreciated that the speed control 63 is provided with such adjustable time-delay circuitries, that momentary variations of the biassin g currents through conductor 62 do not influence the speed control of the mill drum but only prolonged durations from the desired MOG of the product.

The control now described being carried out on a sample of the product discharged from the mill, according to the invention may as well be carried out on a sample of the finished ground product, i.e. the overflow from the cyclone or the classifier in the examples described. The desired MOG is then chosen at a correspondingly lower value.

The invention is not limited to the examples shown and described but may be varied in different ways within the scope of the attached claims defining the invention.

According to the present invention it is possible when comminuting any solid material at a constant feed rate to estimate a suitable combination of composition and size distribution of feed ore and rotational speed which yield a product of the desired MOG and according to the method thereafter automatically to perform the grinding to that MOG.

Having now described the invention, what we claim as new and desire to secure by Letters Patent, is:

l. The method of comrninuting materials by autogenous grinding in a continuous grinding mill to a desired screen analysis, said material varying in dimensions between substantial limits, comprising the steps of feeding a material to be ground to the mill at a substantially predetermined Weight per unit of time, comminuting said material by the autogenous grinding action caused by the interaction of the tumbling mass of bodies to be ground, continuously discharging the ground product from the mill, continuously screening and analyzing the ground product, continuously controlling the screen analysis of the ground product by increasing the r.p.m. of the mill drum when the particle size of the ground product is finer than said desired screen analysis, and by decreasing the rpm. of said mill drum when the particle size of the ground product is coarser than said desired screen analysis.

2. The method of comrninuting materials by autogenous grinding in a continuous grinding mill, said material varying in dimensions between substantial limits, comprising the steps of feeding a material to be ground to the mill at a substantial predetermined Weight per unit of time, comminuting said material by the autogenous grinding action caused by the interaction of the tumbling mass of bodies to be ground, continuously discharging the ground product from the mill, feeding the ground product to a testing screen having a mesh width corresponding to a particle size within the range of the screen analysis of the finished product, screening said ground product to produce a layer of an undersize fraction and a layer of an oversize fraction deposited on the layer of the undersize fraction, continuously producing electrical signals proportional to the thiclmess of the layer of the undersize fraction and the total thickness of the layer of the undersize and oversize fractions, continuously comparing said two signals to produce a first difference signal, continuously producing a reference signal corresponding to the nominal ratio of the predetermined thickness of the layer of the undersize fraction and the total thickness of the layers of the undersize and oversize fractions according to the desired screen analysis of the finished product, com paring said first difference signal with said reference signal to produce a second difference signal, and varying the rotational speed of the mill in accordance with the sense and magnitude of said second diiference signal.

References Cited by the Examiner UNITED STATES PATENTS 2,354,999 8/ 44 Ladd 241-34 2,381,351 8/45 Hardinge 241-35 2,763,790 9/56 Ohmart.

2,766,939 10/56 Weston 241-34 '2, 884,5 31 4/59 Bosch.

I. SPENCER OVERHOLSER, Primary Examiner.

ROBERT A. OLEARY, FRANK H. BRONAUGH,

Examiners. 

1. THE METHOD OF COMMINUTING MATERIALS BY AUTOGENOUS GRINDING IN A CONTINUOUS GRINDING MILL TO A DESIRED SCREEN ANALYSIS, SAID MATERIAL VARYING IN DIMENSIONS BETWEEN SUBSTANTIAL LIMITS, COMPRISING THE STEPS OF FEEDING A METERIAL TO BE GROUND TO THE MILL AT A SUBSTANTIALLY PREDETERMINED WEIGHT PER UNIT OF TIME, COMMINUTING SAID MATERIAL BY THE AUTOGENOUS GRINDING ACTION CAUSED BY THE INTERCATION OF THE TUMBLING MASS OF BODIES TO BE GROUND CONTINUOUSLY DISCHARGING THE GROUND PRODUCT FROM THE MILL, CONTINUOUSLY CONTROLLING THE SCREEN ANALYSIS OF THE PRODUCT, CONTINUOUSLY CONTROLLING THE SCREEN ANALYSIS OF THE GROUND PRODUCT BY INCRESING THE R.P.M. OF THE MILL DRUM WHEN THE PARTICLE SIZE OF THE GROUND PRODUCT IS FINER THAN SAID DESIRED SCREEN ANALSIS, AND BY DECREASING THE R.P.M. OF SAID MILL DRUM WHEN THE PARTICLE SIZE OF THE GOUNND PRODUCT IS COARSER THAN SAID DESIRED SCREEN ANALYSIS. 